1. Field of the Invention
The present invention relates to a semiconductor optical modulator and a method to control optical power output from the same.
2. Related Background Art
The optical communication system using the phase modulation, which drastically enhances the spectrum efficiency, has been proposed. Among the phase modulation techniques, the quadrature phase shift keying (QPSK) combined with the polarization multiplexing, which is called, as the Dual Polarization-Quadrature Phase Shift Keying (DP-QPSK) has been a promising technique.
The DP-QPSK mode requires the even contribution of respective QPSK modulators, one is for X-polarization and the other is for Y-polarization, to the optical power output from the DP-QPSK transmitter. Various techniques to even the contribution have been proposed; however, conventional DP-QPSK modulators have been extremely hard to control the optical power output therefrom in precisely, in particular, the contribution of two QPSK modulators to the optical output power.